Quantum Well Hall Effect (QWHE) sensors and their applications
M. Missous, FREng University of Manchester
PAGE 1 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 INTRODUCTION
Magnetic sensors are fundamental to engineering and the physical sciences and are indispensable components in many systems.
To date, virtually all magnetic sensors are either single elements or a small array of elements operated simultaneously (e.g. 3 component devices or small linear arrays). At best, present technology provides point, area average, or 1-Dimensional (assuming some form of linear translation) measurement of what is in reality a 4-D vector field (3- D space and time).
The ability to capture magnetic field data in more than one dimension would be of real value and would represent a major advance in the state of the art.
Two dimensional arrays of the type proposed here would allow direct high speed vision of magnetic fields, which could have an immediate impact on critical systems, especially those used for inspection or monitoring purposes.
PAGE 2 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Principle of Hall Effect Sensing
When a conductor carrying a current (I) is placed in a magnetic field (B) and oriented so that the current and magnetic field are at right angles, an electric field is produced in the conductor at right angles to both current and magnetic field and produce a Bz I Hall Voltage (Vh) given by: t
Jy Jx
Vh = Kh . B . I Vh
K 1 h t.n.e L
w Where the sensitivity Kh is given by: Fig. 1. Hall effect phenomenon t: Thickness n: Electron concentration e: Electron charge
PAGE 3 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Quantum Well Hall Effect sensors
Fig.2 Formation of 2DEG Quantum Well at AlGaAs/InGaAs heterojunction
2 mm
packaged sensor
2 µm to 70 µm GaAs-InGaAs-AlGaAs QWHE structure and Greek cross geometry with length to width ratio of 3.
PAGE 4 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 SEMICONDUCTOR THIN FILM SYNTHESIS :MOLECULAR BEAM EPITAXY (MBE)
MBE is the ultra high vacuum evaporation of a single crystal oriented overgrowth, from independently controlled constituent species.
RELIES ON UHV TECHNOLOGY
-12 • Ultra clean UHV (pp. O2, H2O, CO, CO2 << 1 x 10 Torr)
PAGE 5 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 EPITAXY
GaAs SUBSTRATE : 500 µm MBE LAYER : 1 µm
YIELD : ~ 40,000 Hall Sensor die per 4” wafer : “All identical”
Ga + As 1022 atoms cm-3 Si (or Be) 1016 atoms cm-3
• Purity : Better than parts per million • Uniformity : < 1% • Precision : < One atomic layer • Time for Deposition : ~ 1 hour
PAGE 6 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Hall Effect Devices Parameters/Limitations
Offset Voltage: Sensitivity Piezoresistance Effect Current sensitivity Si Misalignment. V S h Noise i IB - 1/f Noise - Thermal noise Voltage sensitivity Sv Self Induced Magnetic Field w Sv h G (due to passage of applied current) L
Frequency Limitation Linearity Restricted by relaxation time of Effect of Temperature the dielectric material >> GHz Joule Heating Ambient temperature
PAGE 7 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Magnetic Field Sensors Spectrum
SQUIDs
Nuclear precession magnetometer
Search coil Magnetometers
Fluxgate magnetometer
GMR 2DEG dynamic range > 1010 and no saturation
Commercial GaAs/Si Hall Effect sensor 2DEG Hall Effect sensor 10-6 10-3 10-2 10-1 1 10 102 103 104 105 Gauss 10-10 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10 Tesla
Fig.3. Magnetic field sensor spectrum
PAGE 8 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Solid State Magnetic Sensors : Field sensors and Flux sensors.
The most sensitive magnetic sensors are Flux sensors but at the expense of size (Magnetic induction)
Field sensors measure flux density rather than flux and thus sensitivity is independent of size. (Hall Effect, AMR and GMR)
Field sensors have an inherent advantage in size and power when compared to search coils ,flux gates, and more sophisticated sensing techniques such as Superconducting Quantum Interference Detectors (SQUID).
The sensing can be done in an extremely small, lithographically defined area increasing the resolution for fields that change over small distances and allows for packaging arrays of sensors in a small footprint.
PAGE 9 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 As Hall effect devices are field sensors, their response does not depend on size, and thus micron sized elements can be fabricated.
Due to their small size, these sensors can also be used to field mapping purpose, where a single sensor integrating several elements can be used.
Furthermore the QWHE devices can also be fabricated into transistors therefore enabling integrated circuit configurations with nT/√Hz sensitivity (unlike GMR or AMR).
Unlike Si integrated Hall sensors, the QWHE are radiation resistant and capable of operating in harsh environments ( ~ 200 °C)
PAGE 10 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Noise sources in a QWHE Sensor The noise in a typical semiconductor devices is given by :
α is the Hooge parameter, E is the external electric field applied to the
device, Gn is a geometrical correction factor, f is operating frequency, n2−D is the 2-D electron concentration, Ai,g−r is the amplitude of g−r noise, τi is a characteristic time constant of the generation-recombination process, kB is Boltzmann constant, T is ambient measurement temperature and R the sample resistance.
PAGE 11 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Noise sources at high frequencies
Johnson or resistance noise, dominant above 1 to 10kHz, 1/2 Vn = (4kTBR) , in units of volts/(Sq-root Hz.) k is Boltzman’s constant, k = 1.38 x 10-23 Joule/Kelvin. B = Bandwidth in Hz. R = Resistance in Ohms. T = Temperature in degrees, K. (deg.K = deg.C + 273) The minimum B-field occurs when the S/N ratio is equal to 1 ie
and hence
k is Hall sensitivity and G is magnetic gain ( do not confuse with geometrical factor!).
PAGE 12 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 NOISE MEASUREMENTS
Current Source
Fig. 4 Noise Measurements
PAGE 13 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015
NOISE CHARACTERISTICS Shot Shot Noise
PreAmp : SRS560 DSA : Agilent35670A Current Source : OP27
Fig. 5 Noise characteristics (P2A)
PAGE 14 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Bmin CHARACTERISTICS
P2A QWHE sensor ~ 1µT/ √Hz @ 0.1Hz and 5mA ~ 20 nT/√Hz @ 100kHz and 5mA
Fig. 6 Minimum B-Field detection as a function of frequency
PAGE 15 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Single Sensor applications
PAGE 16 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 1D Passive Magnetic Imaging : Bank Notes Scanning
Using a scanning DC magnetic field mapping system, a £5 and £10 UK and €10 Euro bank notes were scanned.
Position Scanning of £5, £10 UK and €10 bank Figure 7. Processing of scanned bank notes notes raw signals over metal strip , magnetic field strength ~ 5µT
PAGE 17 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 2D passive magnetic imaging
Magnetic disk scan of an 5 x 5 mm2 area, the tracks of the disk are displayed as vertical lines.
Position Floppy disk scanning
Figure 8. Cross section of red line , magnetic flux density variations ~ 10 µT
PAGE 18 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 COMPARISONS WITH GMR and AMR
PAGE 19 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 AMR and GMR sensors
PAGE 20 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 AMR and GMR sensors
Hall Voltage vs Magnetic Flux Density I=1mA
600 500 400 300 200 100 0 Hall Hall Voltage VH (mV) 0 100 200 300 400 500 600 Magnetic Flux Density B (mT)
Die Size: 0.3 x 0.3 mm Die Size: 1.65 x 2.14 mm 40 times smaller than GMR
Maximum operating Temp ~125 C Maximum operating Temp ~200 °C
PAGE 21 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 AMR and GMR sensors
Note : GMR uses FC up to x100
Red line: QWHE sensor Sensitivity (No FC)
Fig. 9 GMR and QWHE sensors sensitivities
PAGE 22 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Combining Hall sensor and flux concentrator
Achieves 100pT/sqrt(Hz) at 1Hz and 1pT/sqrt(Hz) at > 1kHz Magnetic gain ~ 500
20 cm
P. Leroy et al Sensors and Actuators A 142 (2008) 503–510
PAGE 23 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 2 DEG Hall Effect Detection Sensitivity
Detection sensitivity depends on two factors: (1) Scale factor in translating B (Tesla) to output signal voltage and (2) noise that competes with the signal. For 2DEG devices the scale factor is typically 1V/Tesla. The scale factor depends on bias current, which is limited by device dissipation to a few mW at maximum. Dissipation and noise depends on the device resistance.
For our P2A sensor this gives a noise density of 3.4nV/root-Hz above 1kHz, rising at lower frequencies due to flicker noise. A good amplifier will have a noise density in the same region, typically 1-2nV/root-Hz. Obviously to get best sensitivity, we should operate the device with an AC field, if the application allows, and many do allow.
PAGE 24 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 2D Magnetic Field Camera STFC B-Cam PROJECT (with WFS)
PAGE 25 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 DC 2D array magnetometer design
Circuit design DC 2D array magnetometer
Schematic for DC 2D array magnetometer
PAGE 26 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 DC 2D array magnetometer circuit layout
4 Layer PCB layout:
•Top layer: Main components and most tracks for analog circuits
•Mid layer: Most tracks for analog circuits
•Mid-GND layer: Covered by analog ground
•Bottom Layer: Surface mount capacitors and most tracks for digital circuits
PAGE 27 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 DC 2D array magnetometer program flowchart
Program control of the scanning is a left to right cycle
Flowchart of micro-controlled DC 2D Hall array magnetometer
PAGE 28 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 DC 2D array magnetometer prototype
Sensitivity test results of the DC 2D Hall array magnetometer are shown below, where gains of x1, x2, x4, and x8 are the levels with 24, 12, 6, and 3 T icon1, respectively.
DC 2D 8x8x array magnetometer prototype with field visualisation
Sensitivity test results of the DC 2D Hall array magnetometer, where the attenuations in (a), (b), (c), and (d) are 1/32, 1/16, 1/8, and 1/4, respectively
PAGE 29 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 2D Magnetic Field Camera: MFL measurements • Magnetic Flus Leakage (MFL) exploits the fact that magnetic flux lines which are usually confined to the interior of a magnetised material, breaks out (leak) from the confinement at the site of a defect or discontinuity
Test Specimen
N S Magnet Internal Defect External Defect
Flux Leakage
Flux Leakage
PAGE 30 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 MFL Experimental set up
Groove
Groove Specimen
Mild Steel Specimen Bridge (Mild Steel)
(metal background)
PAGE 31 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 3D Magnetic Fields Simulation & Experimental results
Flux Leakage:Simulation and Experiment At the groove boundary
The predicted maximum value of Leakage Field ~ 16 20.0m mT Sim_No Groove Sim_Groove on the top surface 15.0m Expt_Groove on the top surface The experimental maximum value of Leakage Field ~ 9 Expt_No Groove mT
10.0m
5.0m
0.0
Bz (T) -5.0m Groove
-10.0m
-15.0m
-20.0m -4 -3 -2 -1 0 1 2 3 4 Position (cm)
PAGE 32 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Field inversion ( Flaw position)
PAGE 33 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 Field inversion ( Groove position)
PAGE 34 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015 WHERE NEXT ?
2D array in NDT combining coils for scene illumination and 2DEG sensors for image capture (3D Dynamic Magnetic Vision).
Excitation coil
Subject being imaged
Magnetic sensing Detection plate element
PAGE 35 WHERE NEXT ?
Integrated 2D ARRAYS with on board processing
Integrated 2D Sensor~ 2x 2 µm2 to 20 x 20 µm2 or Arrays size ~ larger 1mm x 1mm
Conformal sensing/Mapping Convergence of high performance III-V electronics with Flexible Electronics
AHS
Spin out Company Advanced Hall Sensors (AHS)
PAGE 36 CONCLUSIONS
2DEG AlGaAs/InGaAs/GaAs QW Hall sensors show promise in nanotesla magnetometery.
Field resolutions of ~ 1 T at DC and 20nT at 100kHz ( 1Hz bandwidth and room temperature) are possible.
These Hall effect sensor can be used in banknote validations and magnetic domain imaging.
High resolution 2D arrays are capable of rapid MFL measurements.
3D Dynamic imaging arrays a possibility for Eddy Current testing of conductive and composite materials.
More improvement expected with even more advanced QWHE structures….
PAGE 37 ACKNOWLEDGEMENTS
STUDENTS/PDRAs:
E. Ahmad, E. Balaban, M. Sadeghi, J. Sexton. C. Wei Liang and Z. Zhang
FUNDERS
1. STFC-ST/L000040/1 “HIGH RESOLUTION 2D MAGNETIC VISION- B-Cam “ Nov2013-Oct2016.”
2. TSB Technology Inspired CRD - Advanced Materials " High performance III-V semiconductor materials for magnetic Hall Effect sensors" Nov2013- October2015.
3. EPSRC “UK RESEARCH CENTRE IN NON- DESTRUCTIVE EVALUATION (RCNDE) 2014-2020” EP/L022125/1, Apr2-14-March2020
PAGE 38 THANK YOU AND QUESTIONS?
PAGE 39 Quantum Well Hall Effect (QWHE) sensors and their applications RAL 12th March 2015